2. Topics 4.1 Relational Operators 4.2 The if Statement
4.3 The if/else Statement
4.4 The if/else if Statement
4.5 Menus
4.6 Nested if Statements
4.7 Logical Operators

3. Topics (continued) 4.8 Checking Numeric Ranges with Logical Operators
4.9 Validating User Input
4.10 Comparing Characters and Strings
4.11 The Conditional Operator
4.12 The switch Statement
4.13 Enumerated Data Types
4.14 Testing for File Open Errors

4. 4.1 Relational Operators
Used to compare numbers to determine relative order
Operators:
>
Greater than
<
Less than
>=
Greater than or equal to
<=
Less than or equal to ==
Equal to !=
Not equal to

5. Relational Expressions
Relational expressions are Boolean (i.e., evaluate to true or false)
Examples:
12 > 5 is true
7 <= 5 is false
if x is 10, then
x == 10 is true,
x != 8 is true, and
x == 8 is false

6. Relational Expressions
Can be assigned to a variable
bool result = x <= y;
Assigns 0 for false, 1 for true
Do not confuse = (assignment) and == (equal to)
See pr4-01.cpp

7. 4.2 The if Statement
Allows statements to be conditionally executed or skipped over
Models the way we mentally evaluate situations
“If it is cold outside,
wear a coat and wear a hat.”

8. Format of the if Statement
if (condition) {
statement1;
statement2; …
statementn; }
The block inside the braces is called the body
of the if statement. If there is only 1 statement
in the body, the { } may be omitted.
No ; goes here
; goes here

9. How the if Statement Works
If (condition) is true, then the statement(s) in the body are executed.
If (condition) is false, then the statement(s) are skipped.

10. if Statement Flow of Control
condition
1 or more
statements
true
false

11. Example if Statements if (score >= 60)
cout << "You passed.\n";
if (score >= 90) {
grade = 'A';
cout << "Wonderful job!\n"; }
See pr4-02A.cpp through pr4-02D.cpp for variations on Program 4.2 in the text.

12. if Statement Notes Do not place ; after (condition)
Don't forget the { } around a multi-statement body
Place each statement; on a separate line after (condition), indented
0 is false; any other value is true

13. What is true and false?
An expression whose value is 0 is considered false.
An expression whose value is non-zero is considered true.
An expression need not be a comparison – it can be a single variable or a mathematical expression.

14. Comparisons with floating-point numbers
It is difficult to test for equality when working with floating point numbers.
It is better to use
greater than, less than tests, or
test to see if value is very close to a given value
See pr4-03.cpp and pr4-04.cpp

15. Flags Variables that signals conditions Usually implemented as a bool
Meaning:
true: the condition exists
false: the condition does not exist
The flag value can be both set and tested with if statements

16. Flag Example Example: … if (months < 0) validMonths = false;
bool validMonths = true; …
if (months < 0)
validMonths = false;
if (validMonths)
moPayment = total / months;
See pr4-04.cpp

17. 4.3 The if/else Statement
Allows a choice between statements depending on whether (condition) is true or false
Format: if (condition) {
statement set 1; }
else
statement set 2;

18. How the if/else Works
If (condition) is true, statement set 1 is executed and statement set 2 is skipped.
If (condition) is false, statement set 1 is skipped and statement set 2 is executed.

19. if/else Flow of Control
condition
statement set 1
true
false
statement
set 2

20. Example if/else Statements
if (score >= 60)
cout << "You passed.\n";
else
cout << "You did not pass.\n";
if (intRate > 0)
{ interest = loanAmt * intRate;
cout << interest; }
cout << "You owe no interest.\n";
See pr4-05.cpp and pr4-06.cpp

21. 4.4 The if/else if Statement
Chain of if statements that test in order until one is found to be true
Also models thought processes
“If it is raining, take an umbrella,
else, if it is windy, take a hat,
else, if it is sunny, take sunglasses.”

22. if/else if Format if (condition) { statement set 1; }
else if (condition)
{ statement set 2; …
{ statement set n;
See pr4-07.cpp, compare to pr4-08.cpp

23. Using a Trailing else
Used with if/else if statement when all of the conditions are false
Provides a default statement or action
Can be used to catch invalid values or handle other exceptional situations

24. Example if/else if with Trailing else
if (age >= 21)
cout << "Adult";
else
cout << "Teen";
if (age >= 2)
cout << "Child";
cout << "Baby";
See pr4-09.cpp

25. 4.5 Menus
Menu-driven program: program execution controlled by user selecting from a list of actions
Menu: list of choices on the screen
Can be implemented using if/else if statements

26. Menu-driven Program Organization
Display list of numbered or lettered choices for actions.
Input user’s selection
Test user selection in (condition)
if a match, then execute code to carry out desired action
if not, then test with next (condition)
See pr4-10.cpp

27. 4.6 Nested if Statements
An if statement that is part of the if or else part of another if statement
Can be used to evaluate > 1 data item or condition
if (score < 100) {
if (score > 90)
grade = 'A'; }

28. Notes on Coding Nested ifs
An else matches the nearest if that does not have an else
if (score < 100)
if (score > 90)
grade = 'A';
else ... // goes with second if,
// not first one
Proper indentation helps comprehension
See pr4-11.cpp and pr4-12.cpp

29. Operators, Meaning, and Explanation
4.7 Logical Operators
Used to create relational expressions from other relational expressions
Operators, Meaning, and Explanation
&&
AND
New relational expression is true if both expressions are true || OR
New relational expression is true if either expression is true !
NOT
Reverses the value of an expression; true expression becomes false, false expression becomes true

30. Logical Operator Examples
int x = 12, y = 5, z = -4;
(x > y) && (y > z)
true
(x > y) && (z > y)
false
(x <= z) || (y == z)
(x <= z) || (y != z)
!(x >= z)
See pr4-13.cpp, pr4-14.cpp, and pr4-15.cpp

31. Logical Precedence Example: (2 < 3) || (5 > 6) && (7 > 8)
Highest !
&&
Lowest ||
Example:
(2 < 3) || (5 > 6) && (7 > 8)
is true because AND is evaluated before OR

32. More on Precedence logical operators Lowest relational operators
arithmetic operators
Highest
Example:
8 < || 5 == is true

33. Checking Numeric Ranges with Logical Operators
Used to test if a value is within a range
if (grade >= 0 && grade <= 100)
cout << "Valid grade";
Can also test if a value lies outside a range
if (grade <= 0 || grade >= 100)
cout << "Invalid grade";
Cannot use mathematical notation
if (0 <= grade <= 100) //Doesn’t
//work!

34. 4.8 Validating User Input
Input validation: inspecting input data to determine if it is acceptable
Want to avoid accepting bad input
Can perform various tests
Range
Reasonableness
Valid menu choice
Divide by zero
See pr4-16.cpp

35. 4.9 More About Variable Definitions and Scope
Scope of a variable is the block in which it is defined, from the point of definition to the end of the block
Usually defined at beginning of function
May be defined close to first use
See pr4-17.cpp

36. More About Variable Definitions and Scope
Variables defined inside { } have local or block scope
When in a block that is nested inside another block, you can define variables with the same name as in the outer block.
When in the inner block, the outer definition is not available
Not a good idea
See pr4-18.cpp and pr4-19.cpp

37. 4.10 Comparing Characters and Strings
Can use relational operators with characters and string objects
if (firstName < "Beth")
Comparing characters is really comparing ASCII values of characters
Comparing string objects is comparing the ASCII values of the characters in the strings. Comparison is character-by-character
Cannot compare C-style strings with relational operators
See pr4-20.cpp and pr4-21.cpp

38. 4.11 The Conditional Operator
Can use to create short if/else statements
Format: expr ? expr : expr;
See pr4-22.cpp

39. 4.12 The switch Statement
Used to select among statements from several alternatives
May sometimes be used instead of if/else if statements

40. switch Statement Format
switch (IntExpression) {
case exp1: statement set 1;
case exp2: statement set 2;
...
case expn: statement set n;
default: statement set n+1; }

41. switch Statement Requirements
IntExpression must be a char or an integer variable or an expression that evaluates to an integer value
exp1 through expn must be constant integer type expressions and must be unique in the switch statement
default is optional but recommended

42. How the switch Statement Works
IntExpression is evaluated
The value of intExpression is compared against exp1 through expn.
If IntExpression matches value expi, the program branches to the statement(s) following expi and continues to the end of the switch
If no matching value is found, the program branches to the statement after default:

43. The break Statement Used to stop execution in the current block
Also used to exit a switch statement
Useful to execute a single case statement without executing statements following it

44. Example switch Statement
switch (gender) {
case 'f': cout << "female";
break;
case 'm': cout << "male";
default : cout << "invalid gender"; }
See pr4-23 .cpp, pr4-24.cpp, pr4-25 .cpp, and pr4-26.cpp

45. Using switch with a Menu
switch statement is a natural choice for menu-driven program
display menu
get user input
use user input as IntExpression in switch
statement
use menu choices as exp to test against in the case statements
See pr4-27.cpp

46. 4.13 Enumerated Data Types Data type created by programmer
Contains a set of named constant integers
Format:
enum name {val1, val2, … valn};
Examples:
enum Fruit {apple, grape, orange};
enum Days {Mon, Tue, Wed, Thur, Fri};

47. Enumerated Data Type Variables
To define variables, use the enumerated data type name
Fruit snack;
Days workDay, vacationDay;
Variable may contain any valid value for the data type
snack = orange; // no quotes
if (workDay == Wed) // none here
See pr4-28.cpp

48. Enumerated Data Type Values
Enumerated data type values are associated with integers, starting at 0
enum Fruit {apple, grape, orange};
Can override default association
enum Fruit {apple = 2, grape = 4,
orange = 5} 1 2

49. Enumerated Data Type Notes
Enumerated data types improve the readability of a program
Enumerated variables can not be used with input statements, such as cin
Will not display the name associated with the value of an enumerated data type if used with cout

50. 4.14 Testing for File Open Errors
After opening a file, test that it was actually
found and opened before trying to use it
By testing the file stream object
By using the fail() function

51. Testing the File Stream Object
Example:
ifstream datafile;
datafile.open("customer.dat");
if (!datafile)
cout << "Error opening file.\n";
else
// proceed to use the file

52. Using the fail() Function
Example:
ifstream datafile;
datafile.open("customer.dat");
if (datafile.fail())
cout << "Error opening file.\n";
else
// proceed to use the file